VCSEL Including A Self-Aligned, Deep Hole Evaporated Metal Contact

ABSTRACT

A vertical cavity surface emitting laser (VCSEL) including a first ohmic contact to the substrate formed on an upper surface of the device, instead of the conventional substrate bottom-side contact. The VCSEL is formed to include a hole made through the first distributed Bragg reflector (DBR) and into the material of the substrate itself. A metal layer is deposited at the bottom of the hole to contact the substrate, where the deposited metal layer creates a high quality ohmic contact by not also contacting the inner sidewalls of the hole (i.e., no “stringers” are formed within the hole).

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.14/958,389, which claims benefit of provisional application serial no.62/088,259, filed Dec. 5, 2014, which applications are incorporated byreference herein in their entirety.

BACKGROUND

In semiconductor manufacturing, in microelectronic mechanical systems(MEMS), and in other nanotechnology applications, there frequently is aneed to form a metal contact at the bottom of a relatively deep hole.Unfortunately, the dimensions of the hole and, in particular, its verysmall size and its high aspect ratio (hole depth to hole diameter) oftenpresent challenges to forming good contacts at acceptable yields. Forexample, the sidewalls of the hole often create shadowing effects on thesurface at the bottom of the hole where the contact is to be formed.These challenges are often exacerbated if the hole is formed in amulti-layered structure such as a series of epitaxially grown layers.Such layers are found, for example, in a distributed Bragg reflector(DBR) in a vertical cavity surface emitting laser (VCSEL).

SUMMARY

This invention is a method for forming a metal contact in a relativelydeep hole in a workpiece. In an illustrative embodiment, a first hole isformed in the workpiece at a desired location. The first hole extendsfrom the upper surface of the workpiece to a substrate at the bottom ofthe hole. The first hole is then filled with photoresist by coating theupper surface of the workpiece with a layer of photoresist. Next, aphotolithographic process is performed to create a second hole withinthe photoresist on the sidewalls of the first hole and to expose thesubstrate at the bottom of the second hole. A wet etch is then performedto remove a portion of the substrate at the bottom of the second holeand to undercut some of the photoresist remaining at the bottom of thesecond hole. Next, a layer of contact metal is deposited on the surfaceof the photoresist. This layer is a continuous layer except where thesecond hole is formed. In the second hole, the metal layer is formed onthe exposed surface of the substrate and on discontinuous portions ofthe photoresist on the sidewalls. A liftoff process is then used toremove the photoresist and the metal that was deposited on thephotoresist while leaving the metal at the bottom of the second hole incontact with the substrate.

Advantageously, the foregoing method may be practiced to make a VCSEL inwhich an ohmic contact to the substrate is made through a hole in theepitaxial layers of a DBR.

BRIEF DESCRIPTION OF DRAWING

These and other objects, features and advantages of the invention willbe more readily apparent from the following detailed description inwhich:

FIG. 1 is a cross-section depicting illustrative problems that may beencountered when forming metal layers in relatively deep holes inworkpieces such as semiconductor structures;

FIG. 2 is a flow chart depicting the steps in an illustrative embodimentof the method of the present invention;

FIGS. 3-8 are cross-sections depicting the formation of a metal contactat the bottom of a hole in a VCSEL formed in accordance with theillustrative embodiment of FIG. 2; and

FIG. 9 is a cross-section of a second VCSEL formed in accordance with anillustrative embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 is a cross-section illustrating certain problems that may beencountered when forming an evaporated metal layer at the bottom of arelatively deep hole in a semiconductor structure or othernanostructure. FIG. 1 depicts a workpiece 100 having an upper surface110 in which there is a first hole 130 that extends downward from theupper surface to a lower surface 120. Hole 130 has sidewalls 135 inworkpiece 100. A layer 140 of photoresist covers upper surface 110 ofthe workpiece and sidewalls 135 of hole 130. The layer of photoresistalso has a second hole 160 that is centered in hole 130 and extends fromthe upper surface 150 of the photoresist to lower surface 120 of theworkpiece. A mask 170 covers the upper surface of the layer ofphotoresist except for a third hole 180 above second hole 160 in thephotoresist The mask includes an overhang 175 that extends over aportion of second hole 160.

As shown in FIG. 1, when a metal layer is evaporated onto the lowersurface of the hole to form a metal contact, the sidewalls of the holeoften create shadowing effects 190 on the surface at the bottom of thehole. These effects often limit the size or the quality of the contactthat can be formed. The overhang 175 of the mask may also contribute tothese shadowing effects. Conversely, if the overhang is not greatenough, metal stringers 195 may form on the sidewalls of the hole. Suchstringers also impair the quality of the metal contact.

FIG. 2 is a flow chart depicting an illustrative embodiment of a method200 for forming a metal contact that alleviates these problems. Forconvenience of description, the method will first be described for thecase of formation of a metal contact at the bottom of a hole in ageneric workpiece. Examples of the formation of a contact at the bottomof a hole in a VCSEL will then be provided in FIGS. 3-9.

Method 200 begins at step 210 as a first hole is formed at a desiredlocation in a workpiece. The first hole extends from an upper surface ofthe workpiece to a substrate at the bottom of the hole. At step 220, thefirst hole is filled with photoresist by coating the upper surface ofthe workpiece with a layer of photoresist. At step 230, aphotolithographic process is then performed to create a second holewithin the photoresist on the sidewalls of the first hole and expose thesubstrate at the bottom of the second hole. Illustratively, thephotolithographic process includes the placement on the photoresist of amask that defines selected portions of the photoresist that are to beremoved, selective exposure of the photoresist by actinic radiationdirected through the mask, and removal of the selected portions of thephotoresist.

A wet etch is then performed at step 240 to remove a portion of thesubstrate at the bottom of the second hole and to undercut some of thephotoresist covering an outer perimeter of the first hole, thisphotoresist perimeter labeled as “P” in FIGS. 5-7. Next, at step 250, alayer of contact metal is deposited on the surface of the photoresist.This layer is a continuous layer except where the second hole is formed.In the second hole, the metal layer is formed on the exposed surface ofthe substrate and on photoresist perimeter P. A liftoff process is thenperformed at step 260 to remove the photoresist and the metal that wasdeposited on the photoresist while leaving the metal at the bottom ofthe second hole in contact with the substrate. In accordance with themethod of the present invention, therefore, the photoresist remaining onthe sidewalls of the first hole will prevent the deposited metal fromcontacting the sidewalls, thereby eliminating the possibility of formingmetal stringers in the inventive process.

FIGS. 3-8 are cross-sections of a hole depicting the formation of themetal contact in accordance with the steps of FIG. 2. Illustratively,these steps are performed in a series of epitaxially grown layers suchas found in a distributed Bragg reflector (DBR) in a vertical cavitysurface emitting laser (VCSEL). FIG. 3 depicts an illustrativeembodiment 300 of a VCSEL comprising a substrate 310, a firstdistributed Bragg reflector (DBR) 320 on a first major surface of thesubstrate, an active region 330 on the first DBR, and a second DBR 340on the active region. Each DBR is a set of alternating layers of twosemiconductor materials having different indices of refraction with eachlayer having a thickness of one quarter the operating wavelength of theVCSEL. Optical interference between the radiation reflected at theinterface between successive layers makes each DBR a highly effectivereflector. DBRs 320 and 340 form a laser cavity, and when a suitablecurrent is established through the VCSEL, laser emission takes place.

Typically, the current is confined to a narrow region in the VCSEL bylimiting the lateral dimensions of the active region and the second DBRand/or by inserting an aperture between the active region and the secondDBR. In the embodiment shown in FIGS. 3-8, current confinement isachieved by forming a trench isolation region 360 around the activeregion and the second DBR and forming an oxide aperture 370. It will beappreciated that the alternating layers of semiconductor material extendthe full width of the structure shown in FIGS. 3-8 even though thelateral dimensions of the active region and the second DBR are only asmall part of the lateral extent of the first DBR.

Ohmic electrical contacts to the VCSEL are typically made to thesubstrate and the second DBR. This, however, is often a problem since itfrequently is desirable to have the electrical leads for both contactslocated on the same side of the VCSEL. To contact the substrate from thesame side of the VCSEL as the second DBR, it is necessary to form a hole380 in the epitaxial layers that extends from an upper surface of theVCSEL down to the substrate and to form a metal contact on the exposedsurface of the substrate at the bottom of the hole.

As shown in FIG. 4, the first hole is then filled with a photoresist 410by coating the upper surface of the VCSEL with a layer of photoresist.As shown in FIG. 5, a photolithographic process is then performed tocreate a second, smaller diameter hole 510 through the photoresist 410within the first hole, the second, smaller diameter hole 510 formed toexpose a portion 520 of substrate 310 at the bottom of the second hole.As shown in FIG. 6, a wet etch is then performed to remove a portion 620of the substrate at the bottom of the second hole and to undercut someof the photoresist perimeter P covering an outer region of the firsthole. Next, as shown in FIG. 7, a layer of contact metal 710 isdeposited on the surface of the photoresist. This layer is a continuouslayer except where the second hole is formed. In the second hole, themetal layer is formed on the exposed surface 720 of the substrate and ondiscontinuous, non-vertical portions of the photoresist perimeter Pcovering the outer region of the first hole. A liftoff process is thenperformed that removes the photoresist and the metal that was depositedon the photoresist while leaving the metal at the bottom of the secondhole in contact with the substrate as shown in FIG. 8, where asdiscussed above the step of maintaining the photoresist on the sidewallsof the hole until the metal is deposited prevents metal stringers fromforming along the sidewalls (as occurred in prior art processes). Anelectrical lead is then formed in the hole to connect to the metal; andanother ohmic contact (not shown) is formed on the second DBR.

FIG. 9 depicts a second illustrative embodiment of a vertical cavitysurface emitting laser (VCSEL) 900. VCSEL 900 comprises a substrate 910,a first distributed Bragg reflector (DBR) 920 on a first major surfaceof the substrate, an active region 930 on the first DBR, and a secondDBR 940 on the active region. Illustratively, these elements are shapedin a so-called mesa or air-post configuration with the lateraldimensions of the active region, second DBR and second ohmic contactbeing appreciably smaller than the lateral dimensions of the first DBRand substrate.

To contact substrate 910 from the same side of the VCSEL as theconnection to second DBR 940, a hole 980 is formed in the epitaxiallayers of the first DBR on the portion of the first DBR that is notcovered by the active region and second DBR. Hole 980 extends from anupper surface of the first DBR down to substrate 910 and a metal layer985 makes ohmic contact to the exposed surface of the substrate at thebottom of the hole. An electrical lead 987 extends through the hole toconnect to metal layer 985. Lead 988 is electrically insulated from theepitaxial layers of first DBR 920 by a suitable insulating layer 989. Asecond metal layer 990 makes an ohmic contact with second DBR 940.

Hole 980 and metal layer 985 may be formed using the same steps asdescribed in conjunction with FIG. 2.

As will be apparent to those skilled in the art, numerous variations maybe practiced within the spirit and scope of the present invention.

What is claimed is:
 1. A vertical cavity surface emitting layer (VCSEL)comprising: a substrate having first and second major surfaces; a firstdistributed Bragg reflector (DBR) disposed on the first major surface ofthe substrate; an active region disposed on a top surface of the firstDBR; a second DBR disposed over the active region; a hole that extendsthrough at least the first DBR and into the substrate, where a bottomsurface of the hole exposes a portion of the substrate; a first metallayer formed to cover a major extent of the bottom surface of the hole,the coverage of the first metal layer sufficient to form a first ohmiccontact with the substrate; a first electrically conductive materialextending through the hole to make electrical contact between the firstmetal layer and the first DBR; and a second ohmic contact coupled to thesecond DBR.
 2. The VCSEL of claim 1 wherein the active region and thesecond DBR have a lateral extension that is less than that of the firstDBR, exposing a portion of the top surface of the first DBR, with thehole formed in the exposed portion of the first DBR.
 3. The VCSEL ofclaim 2 wherein the active region and the second DBR comprise a mesageometry.
 4. The VCSEL of claim 1 further comprising an aperturedisposed between the active region and the second DBR.
 5. The VCSEL ofclaim 4 wherein the aperture confines current flow in the VCSEL.
 6. TheVCSEL of claim 1 wherein the first DBR comprises a plurality ofepitaxially grown layers.
 7. The VCSEL of claim 1 wherein the firstmetal layer does not overlap with a sidewall of the hole.
 8. The VCSELof claim 1 wherein the VCSEL further comprises an insulating layerdisposed between the first electrically conductive material and thefirst DBR.
 9. The VCSEL of claim 8 where the insulating layer isdisposed vertically along an inner wall of the hole from the first metallayer to a contact area on the first DBR.